I don't think that it is possible to be a completionist in Gothic 3 because of its structure and factions. For example, if you align yourself totatly with the orcs then you will not be able to work with the bandits (and nordmarians) and probably miss their quests. Likewise if you align yourself with the bandits then you will not be able to proceed with the orcs and so on. That is why I think that Gothic 3 has an amazing replay value (not to mention the easter eggs scattered around the huge map).

I played it first time using cp 1.74 and it took me about 170 h to complete it at level 75 and I have missed a large number of quests.